The reaction of isocyanate and hydroxyl compounds to form urethanes is the basis for the production of polyurethanes. Metal compounds (e.g., tin, zinc and bismuth compounds) and tertiary amines have been known to catalyze the reaction of isocyanate and hydroxyl groups to form urethane. See, Proceedings of Water Borne and High Solids Coatings Symposium, Feb. 25-27, 1987, New Orleans, at Page 460. Compounds useful for the isocyanate-hydroxyl reaction are also referred to as urethane catalysts. At present, the commercially available catalysts used in this reaction are organotin compounds (e.g., dibutyltin dilaurate and dibutyltin diacetate), zinc carboxylates, bismuth carboxylates, organomercury compounds and tertiary amines.
There are several problems with these commercially available catalysts. When they are used in the process for polyurethane coatings, the cure of the coatings under high humidity or at low temperature conditions is not satisfactory. They catalyze the undesirable side reaction of isocyanate with water to form amines and carbon dioxide. The carbon dioxide may cause blisters in the coating and the amines react with isocyanates resulting in low gloss coatings. Moreover, the cure rate at low temperatures is too slow. The commercially available catalysts also catalyze the degradation of the resulting polymer product. Furthermore, several of the commercially available urethane catalysts, particularly those containing heavy metals and tertiary amines, are highly toxic and are environmentally objectionable.
The testing of zirconium acetylacetonate and zirconium tetra-3-cyanopentanedionate, as catalysts for the isocyanate-hydroxyl reaction have been described in GB Patents 908949, 890280 and 869988. Subsequent testing by others, however, has shown that zirconium acetylacetonate is a poor catalyst for the urethane reaction. B. D. Nahlovsky and G. A. Zimmerman, Int. Jahrestag. Fraunhofer—Inst. Treib-Explosivst., 18th (Technol. Energ. Mater.), 39:1-12, reported that the catalytic efficiency of zirconium acetylacetonate for the isocyanate-hydroxyl reaction to form urethane is low. The solubility of zirconium acetylacetonate and zirconium tetra-3-cyanopentanedionate in solvents commonly used in the production of coatings is poor. Examples of such solvents include esters ketones, glycolesters and aromatic hydrocarbons, such as: butyl acetate, methyl iso-amyl ketone, 2-methoxy propylacetate, xylene and toluene. Because of the low catalytic efficiency and the poor solvent solubility, the use of these compounds as catalysts in processes involving urethane or polyurethanes have been limited.
Further testing using zirconium acetylacetonate in our laboratory has shown that zirconium compounds disclosed in the prior art, will only catalyze the isocyanate-hydroxyl reaction when carried out in a closed system, i.e., in a closed pot. This is impractical for many of the polyurethane applications. The zirconium diketonates of the prior art failed as catalysts when the reaction is carried out in the open atmosphere, unless there is present a large excess of the corresponding diketone. For zirconium acetylacetonate, the presence of over 1000 to 1 mole ratio of 2,4-pentanedione to zirconium acetylacetonate is required. However, 2,4-pentanedione and other similar diketones are volatile solvents which, when used in an open vessel, pollute the air, and pose both an environmental and a fire hazard. In addition, the presence of the free diketone causes discoloration of the catalyst, resulting in an undesirable, discolored product.
Blocked isocyanates have been used in many coating applications, such as powder coatings, electrocoatings, coil coatings, wire coatings, automotive clear top coatings, stone chip resistant primers, and textile finishes. Traditionally, these coating processes employ organic solvents, which may be toxic and/or obnoxious and cause air pollution. In recent years, the legal requirements for low or no pollution of the environment have led to an increase in the interest in waterborne and high solids coatings.
In processes wherein blocked isocyanates are used, heating to an elevated temperature is necessary to remove the blocking group from the blocked isocyanate to form free isocyanates. The free isocyanates then react with polyols (polymers containing hydroxyl functional groups) to form a crosslinked network as a thin film coating. An obstacle to the use of this process is the high temperature required to remove the blocking group. The process is extremely slow without a catalyst. It is known that metal compounds such dialkyltin and certain bismuth and zinc salts are excellent catalysts in these solvent borne coating processes. “Crosslinking with Polyurethanes.” W. J. Blank, ACS Proceedings of Polymeric Materials Science and Engineering (1990) 63:931-935.
Bismuth organo-compounds have been used in a variety of processes wherein polyisocyanates or blocked isocyanates is an ingredient. For example, EP 95-109602 describes an epoxide amine adduct with a bismuth compound as being useful in a conventional cationic coating process. U.S. Pat. No. 5,702,581 describes the use of organic bismuth complexes in phosphate dip coating compositions to provide corrosion resistance. The bismuth organic complexes include bismuth carboxylates, such as bismuth lactate. WO 95/29007 disclosed the use of bismuth compounds/mercapto complexes for curing polyisocyanate organic solvent compositions. The bismuth compounds disclosed include bismuth carboxylates, nitrates and halides. WO 96/20967 also described bismuth/zinc mixture with a mercapto complex as a catalyst for producing polyurethane. See also Frisch et al., “Novel Delayed-Action Catalyst/Co-catalyst system for C.A.S.E. Applications”, 60 Years Polyurethanes, Kresta et al. ed., Technomic: Lancaster, Pa. 1998, pp. 287-303. Further, WO 95/08579 described bismuth/mercapto complexes as latent catalysts in a polyol-polyisocyanate adhesive system. The catalyst is described as useful in promoting the rapid cure of the system. The bismuth carboxylates described in these references are those wherein the carboxylate has ten carbons or less in the hydrocarbon structure. These conventional bismuth carboxylates do not provide improved resin performance nor are they effective in water-borne formulations.
WO 95/07377 described the use of bismuth lactate in cationic lacquer compositions, which employ urethane reactions. A mixture of bismuth and an amino acid or amino acid precursor was disclosed for catalyzing a cationic electrodeposition of a resin film on a metal substrate. The bismuth may be present in the form of nitrates, oxides, trioxides, or hydroxide. DE 19,532,294A1 also disclosed bismuth carboxylates as catalysts for single component polyurethane lacquer coatings in a solvent borne formulation.
Unfortunately, when the known bismuth catalysts are employed in waterborne coatings formulations, it was found that they were not effective. It is suspected that the loss of activity is related to the hydrolysis of the bismuth salt in water. Moreover, even if these compounds function as catalysts in waterborne processes, it has been our experience that a very high level is necessary, usually 10 to 100 times higher than in solvent borne processes. This is undesirable because it would cause environmental pollution if a large amount is released into the environment.
Bismuth carboxylates have been used as catalysts in processes that do not involve de-blocking of blocked isocyanates. Bismuth dimethylol propionate has been disclosed in DE 93-43,300,002 as being useful in an electrocoating process for coating phosphate dipped metals to provide anti-corrosion and weather resistance. Bismuth carboxylates are also described in DE 96-19,618,825 for use in an adhesive gel formulation that is safe for contact with human skin. The formulation contains polyether polyols with hydroxyl groups, antioxidants, Bismuth(III) C2-C18 carboxylates soluble in the polyether polyols and OCN(CH2)6NCO. JP 95-351,412 describes the use of bismuth neodecanoate as a catalyst for two part adhesive formulations containing polyisocyanates, polyols with an ethylenediamine. These formulations do not involve the de-blocking of blocked isocyanates.
For waterborne processes, the catalysts known to be useful are organo-tin and lead compounds. See WO 95/04093, which describes the use of organo-tin alone or in a mixture with other compounds including bismuth oxide in a low temperature curing process employing blocked isocyanates. There is no disclosure of bismuth carboxylates alone as a catalyst for de-blocking isocyanates. Organo-tin compounds have also been used in coatings, e.g. in paints for anti-fouling applications. Organo-tin compounds in mixtures with bismuth hydroxyl carboxylic acid salt was described in DE19,613,685. The use of bismuth lower carboxylates was described as being useful in a phosphate dip process to provide corrosion resistance to lacquer coatings. The bismuth carboxylates described therein as being useful are lower carboxylate of bismuth wherein the carboxylic acid has up to ten carbons. The substrate is then coated with an epoxy resin in the presence of a blocked isocyanate as the crosslinking agent using a zinc organo compound and/or lead compound as the catalyst. EPO, 509,437 disclosed a mixture of a dibutyltin aromatic carboxylate with bismuth and a zirconium compound as the dissociation catalyst for electrocoating wherein a blocked isocyanate is used. Polystannoxane catalysts are also described in EPO, 810,245 A1 as an low temperature catalyst for curing compositions comprising a blocked isocyanate. Bismuth compounds, including carboxylates were described as being useful as a co-catalyst. However, the process is one in which the reaction temperature was in the range of 100° C., quite a bit below the normal temperature of 120° C. to 150° C. for de-blocking blocked polyisocyanates. JP 94-194950 described a formulation for coating materials which are rapidly curable in contact with an amine catalyst vapor or mist. The coating formulation included polyols, polyisocyanates, and antimony or bismuth catalysts with mercaptans in an organic solvent. The toxicity of both lead and tin compounds presents serious environmental hazards. The use of solvents in solvent borne processes further result in the undesirable release of toxic and obnoxious chemicals into the environment. For these reasons, the use of organo tin and lead compounds and solvents has been banned in many applications and is highly restricted in electrocoating.
It is, therefore, important to develop other catalysts or catalysts systems for waterborne processes.
As environmental legislation has become ever stricter, the development of powder coatings, together with high solids lacquers and aqueous coating systems has become increasingly significant in recent years. Powder coatings release no harmful solvents during application, may be applied highly efficiently with little waste and, thus, are considered particularly environmentally friendly and economic.
Particularly high quality light and weather resistant coatings may be obtained using heat curable, polyurethane (PUR) powder coatings. The PUR powder coatings currently commercially available generally contain solid polyester polyols, which are cured with solid blocked aliphatic or, usually, cycloaliphatic polyisocyanates. However, these systems exhibit the disadvantage that the compounds used as blocking agents are released during thermal crosslinking. As a consequence, particular precautions must be taken during application both for equipment-related reasons and for environmental and occupational hygiene reasons to purify the exhaust air and/or to recover the blocking agent.
One approach to avoiding the emission of blocking agents is to use known PUR powder coating crosslinking agents containing uretdione groups as described, e.g., in DE-A 2,312,391, DE-A 2,420,475, EP-A 45,994, EP-A 45,996, EP-A 45,998, EP-A 639,598 and EP-A 669,353. These products crosslink by the thermal dissociation of uretdione groups into free isocyanate groups and the subsequent reaction of these groups with the hydroxyl-functional binder. In practice, however, uretdione powder coating crosslinking agents have only been used on an infrequent basis. The reason for this resides in the relatively low reactivity of the internally blocked isocyanate groups, which generally require stoving temperatures of at least 160° C.
Although it is known that the uretdione cleavage reaction is noticeable at temperatures as low as 100° C., especially in the presence of reactants containing hydroxyl groups, the reaction proceeds so slowly at this temperature that complete curing of coatings would take several hours, an unrealistically long period for practical use. DE-A 2,420,475, DE-A 2,502,934 or EP-A 639,598 mention temperatures as low as 110° C., or even as low as 90° C. (DE 2,312,391), as possible stoving conditions for powder coating systems containing uretdione groups. However, the examples demonstrate that even with the powder coatings described in these publications, adequately crosslinked coatings are only obtainable at temperatures of 150° to 160° C. within practical stoving times of at most 30 minutes. Hydroxyl-terminated polyaddition compounds containing uretdione groups, as described in EP 0 669 353, require a very high baking temperature of at least 180° C., for adequate cure.
Several approaches to accelerate the curing of uretdione-crosslinking PUR powder coatings using various catalysts, have been exemplified inn various patents/publications. Several compounds have already been proposed for this purpose, for example, the organometallic compounds known from polyurethane chemistry, such as tin(II) acetate, tin(II) octoate, tin(II) ethylcaproate, tin(II) laurate, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate (for example EP 803 524, EP-A 45,994, EP-A 45,998, EP-A 601,079, WO 91/07452 or DE-A 2,420,475), iron(III) chloride, zinc chloride, zinc 2-ethylcaproate and molybdenum glycolate or tertiary amines such as triethylamine, pyridine, methylpyridine, benzyldimethylamine, N,N-endoethylenepiperazine, N-methylpiperidine, pentamethyldiethylenetriamine, N,N-dimethylaminocyclohexane and N,N′-dimethylpiperazine (for example EP-A 639 598), N,N,N′-trisubstituted amidines (U.S. Pat. No. 5,847,044), tetra alkyl ammonium compounds and combinations with reactive compounds that are able to react with acid groups, (E. Spyrou, H. Loesch, and J. V. Weiβ, “Highly Reactive, Blocking Agent-Free Polyurethane Powder Coatings”, 8th Nuernberg Congress, Creative Advances in Coatings Technology; Nuernberg, Germany, April, 2005, U.S. Pat. No. 6,914,115 B2), and metalloorganic carboxylate, alcoholate, or acetylacetonate and combinations of these catalysts with reactive agents such as an epoxy, or an oxazoline compound (WO 00/34355, U.S. Pat. No. 7,019,088 B1).
EP 803 524 also mentions other catalysts which have been used to date for this purpose, but without showing any particular effect on the curing temperature. They include the organometallic catalysts known from polyurethane chemistry, such as dibutyltin dilaurate (DBTL), for example, or else tertiary amines, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), for example.
While EP-A 652,263, which describes the use of powder coating curing agents containing uretdione groups as an additive for powder coating compositions based on epoxy-functional copolymers and carboxyl derivatives as the crosslinking agent, do make a general reference to the two amidine bases DBN and 1,8-diazabicyclo(5.4.0) undec-7-ene (DBU) in a lengthy list of curing catalysts, the person skilled in the art could not gain any concrete indication from this disclosure that precisely these two compounds are highly effective catalysts for the dissociation of uretdione rings. This is because the working examples do not use these two catalysts, but instead an organometallic catalyst as is conventional in known PUR powder coating compositions containing uretdione groups. This reference does not recognize that the catalysts according to the present invention are particularly effective for uretdione dissociation. The low stoving temperatures for the powder systems described in EP-A 652,263 are not attributable to uretdione cleavage accelerated by catalysis with amidine bases, but are in fact within the usual range for epoxy/dicarboxylic acid systems.
While U.S. Pat. No. 5,847,044 describes a polyurethane powder coating composition, containing uretdione groups and describes the use of N,N,N′-trisubstituted amidine catalysts, this reference does not recognize that the catalysts of the present invention are effective. Although N,N,N′-trisubstituted amidine catalysts lead to a reduction in the curing temperature, they exhibit a marked yellowing, which is generally unwanted in the coatings field. The cause of this yellowing is probably the reactive nitrogen atoms in the amidines. These can react with atmospheric oxygen to give N-oxides, which are responsible for the discoloration. It is also noteworthy that the compositions of the present invention lead to coatings with no yellowing unlike the compositions when amidine bases are used as the catalyst.
WO 00/34355 claims catalysts based on metal acetylacetonates, e.g., zinc acetylacetonate. Such catalysts are in fact able to lower the curing temperature of polyurethane powder coating compositions containing uretdione groups, but as reaction products give primarily allophanates (M. Gedan-Smolka, F. Lehmann, D. Lehmann, “New catalysts for the low temperature curing of uretdione powder coatings” International Waterborne, High solids and Powder Coatings Symposium, New Orleans, Feb. 21-23, 2001). Allophanates are the reaction products of one mole of alcohol and two moles of isocyanate, unlike conventional urethanes which result from the reaction of one mole of alcohol with one mole of isocyanate, leading to increased cost of the overall system.
It has been long recognized that epoxy compounds react with carboxylic acids or with anhydrides. It is also known that this reaction can be catalyzed. Antoon and Koenig (J. Polym. Sci., Polym. Chem. Ed. (1981) 19(2):549-70) studied the mechanism of catalysis by tertiary amines of the reaction of anhydrides with epoxy resins, typically a glycidyl ether of bisphenol A. They pointed out that it is the quaternary ammonium salt zwitterion that initiated the polymerization reaction. Matejka and Dusek studied the reaction of phenylglycidyl ether model compounds with caproic acid in the presence of a tertiary amine as the catalyst (Polym. Bull. (1986) 15(3):215-21). Based on their experimental data, they suggested that this is an addition esterification process.
Metal salts and amines have been used as catalysts for the epoxycarboxyl/anhydride reaction. For example, 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), a strong basic amine and its salts are being promoted as catalysts for epoxy-carboxyl/anhydride polymer systems. It is known that the salts of amines usually improved the pot life of such polymer systems. Whittemore et. al. (U.S. Pat. No. 3,639,345) disclosed thermosetting resins using an epoxy functional bisphenol A and a trimellitic anhydride ester with an amine, an imidazole or an aminoalkyl phenol, as the catalyst.
Metal salts or Lewis acid catalysts are also promoted for epoxy resins. The metal salts have found applications as catalysts for epoxycarboxyl/anhydride coatings. The catalytic effect of metal salts was recognized by Connelly et. al. (ZA U.S. Pat. No. 6,907,152) who described the use of zinc acetate, chromium acetate, iron octoate, zinc naphthenate, cobalt naphthenate and manganese naphthenate as catalysts. Metal salts of Mg, Ca, Sr, Ba, Zn, Al, Sn, and Sb have been disclosed by Lauterbach (U.S. Pat. No. 4,614,674) as catalysts in combination with waxes as matting agents for powder coatings. Wright et. al. disclose (U.S. Pat. No. 4,558,076) a fast curing coating formulation comprising a carboxyl functional polymer, a tertiary amine, a polyepoxide and an Al, Ti, or Zn alkoxide or complex as the catalyst.
A major problem with the known catalysts is the poor stability of the combination of the epoxy and carboxyl/anhydride reactants at ambient room temperature. The increase in viscosity requires the epoxy and the carboxyl/anhydride compounds to be formulated into two separate packages. A further problem is the yellowing tendency of amines during the bake or heating cycle. In addition, it is known that the use of amines result in films that are sensitive to humidity leading to blistering of the film. It would be desirable to have a catalyst that does not require the separate packaging of epoxy and carboxyl/anhydride reactants and does not cause yellowing or sensitivity to humidity leading to blistering.
Metal salts such as zinc carboxylates have been shown to be effective catalysts in the above referenced patents. However, the problem with di and polyvalent metal salts is salt formation with the carboxyl groups of the reactant through ionic crosslinking leading to an instant increase in viscosity or gelation. Although covalent bonds are not formed in this process, this reaction can lead to very highly viscous formulations with poor flow quality resulting in poor film properties.
Zinc and cadmium complexes with N-substituted imidazoles have been described by Pettinari et al. (Polyhedron, (1998), 17(10):1677-91). The complexes described are in the hydrate form and there is no discussion or suggestion of their use as catalysts in the production of polyurethane or epoxy based polymer coatings.